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  初识
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          英语读音
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  音标
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  Java知识路线
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  工作
</a></li></ul></div> <!----></div> <!----></nav>  <ul class="sidebar-links"><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading open"><span>Java 基础系列</span> <span class="arrow down"></span></p> <ul class="sidebar-links sidebar-group-items"><li><a href="/java/basics/know.html" class="sidebar-link">Java-初识</a></li><li><a href="/java/basics/number.html" class="sidebar-link">Java-数字</a></li><li><a href="/java/basics/string.html" class="sidebar-link">Java-字符串</a></li><li><a href="/java/basics/array.html" class="sidebar-link">Java-数组</a></li><li><a href="/java/basics/error.html" class="sidebar-link">Java-异常处理</a></li><li><a href="/java/basics/generics.html" aria-current="page" class="active sidebar-link">Java-泛型</a></li><li><a href="/java/basics/reflect.html" class="sidebar-link">Java-反射</a></li></ul></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Java 集合</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Java 线程</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Java IO</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Java 数据库</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Java 网络编程</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Java 常用包</span> <span class="arrow right"></span></p> <!----></section></li></ul> </aside> <main class="page"> <div class="main"><div class="content"><div class="theme-default-content content__default"><h1 id="java-泛型"><a href="#java-泛型" class="header-anchor">#</a> Java-泛型</h1> <p></p><div class="table-of-contents"><ul><li><a href="#前言">前言</a></li><li><a href="#泛型类">泛型类</a></li><li><a href="#泛型方法">泛型方法</a></li><li><a href="#泛型集合">泛型集合</a></li><li><a href="#泛型接口">泛型接口</a></li><li><a href="#泛型的限制">泛型的限制</a><ul><li><a href="#通配符">通配符</a></li><li><a href="#extends与super">extends与super</a></li></ul></li></ul></div><p></p> <p>🐴</p> <h2 id="前言"><a href="#前言" class="header-anchor">#</a> 前言</h2> <p><code>Java</code> 泛型（<code>generics</code>）是 <code>JDK 5</code> 中引入的一个新特性, 泛型提供了编译时类型安全检测机制，该机制允许程序员在编译时检测到非法的类型。在<code>java</code>集合中，存入对象后，在取出时该对象时，该对象就会成为<code>Object</code>类型。这时我们使用泛型就可以避免这种情况。例如下面这个例子：</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token class-name">List</span> arrayList <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
arrayList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">&quot;12121&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
arrayList<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token keyword">for</span><span class="token punctuation">(</span><span class="token keyword">int</span> i <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span> i <span class="token operator">&lt;</span> arrayList<span class="token punctuation">.</span><span class="token function">size</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> i<span class="token operator">++</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
  <span class="token class-name">String</span> s <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">)</span>arrayList<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span>i<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>上面<code>ArrayList</code>集合中存入了<code>String</code>类型和<code>integer</code>类型，都可以存入是因为，存入时把它们当作<code>Object</code>数据存入的。结果我们取值时，都按照<code>String</code>来取的，这时程序就会报下面错误：</p> <p><code>java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String</code></p> <p>为了避免出现类似的问题，我们可以使用<strong>泛型</strong></p> <p><code>List&lt;String&gt; arrayList = new ArrayList&lt;String&gt;();</code></p> <p>这时我们在想向<code>arrayList</code>中存入<code>integer</code>类型时，程序是不通过的。下面汇总了<code>java</code>中泛型的具体用法</p> <h2 id="泛型类"><a href="#泛型类" class="header-anchor">#</a> 泛型类</h2> <p>泛型在类中使用时，我们称其为<strong>泛型类</strong>，如下：</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token comment">// T 为泛型标识，可以改成任意字母，</span>
<span class="token comment">// 常见的用 T E K 等来标识泛型</span>
<span class="token keyword">public</span> <span class="token keyword">class</span> test<span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">{</span>
   <span class="token keyword">private</span> <span class="token class-name">T</span> a<span class="token punctuation">;</span>
   <span class="token keyword">public</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token class-name">T</span> a<span class="token punctuation">)</span><span class="token punctuation">{</span>
     <span class="token keyword">this</span><span class="token punctuation">.</span>a <span class="token operator">=</span> a<span class="token punctuation">;</span>
   <span class="token punctuation">}</span>
   <span class="token keyword">public</span> <span class="token class-name">T</span> <span class="token function">getA</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
     <span class="token keyword">return</span> a<span class="token punctuation">;</span>
   <span class="token punctuation">}</span>

  <span class="token comment">// 使用的时候必须指定T的类型</span>
  <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token class-name">Test</span> tStr <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Text</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token string">&quot;a&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// T 为 String 类型</span>
    <span class="token class-name">Test</span> tInt <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Text</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token number">123</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// T 为 integer 类型</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><h2 id="泛型方法"><a href="#泛型方法" class="header-anchor">#</a> 泛型方法</h2> <p>泛型方法一般类似这样<code>public &lt;T&gt; void method(T t){}</code> 带有<code>&lt;T&gt;</code>。具有<code>&lt;T&gt;</code>标识是很重要的，这表示该方法声明为泛型方法。常见的泛型方法类中的泛型方法，静态泛型方法，如下</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> test<span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span> <span class="token punctuation">{</span>
  <span class="token comment">// 泛型方法 该方法中的T 和 类 test 中的</span>
  <span class="token comment">// T 是不一样的。此处的T 表示一种新的类型</span>
  <span class="token keyword">public</span> <span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span> <span class="token keyword">void</span> <span class="token function">funOne</span><span class="token punctuation">(</span><span class="token class-name">T</span> t<span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>t<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
  <span class="token comment">// 泛型方法类型可以不适用T，这里使用E来表示</span>
  <span class="token keyword">public</span> <span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">E</span><span class="token punctuation">&gt;</span></span> <span class="token keyword">void</span> <span class="token function">funTwo</span><span class="token punctuation">(</span><span class="token class-name">E</span> e<span class="token punctuation">)</span><span class="token punctuation">{</span>
     <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>e<span class="token punctuation">.</span><span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
  <span class="token comment">// 有返回值的泛型方法</span>
  <span class="token keyword">public</span> <span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">E</span><span class="token punctuation">&gt;</span></span> <span class="token class-name">E</span> <span class="token function">funThree</span><span class="token punctuation">(</span><span class="token class-name">E</span> e<span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">return</span> e<span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
  <span class="token comment">// 静态泛型方法</span>
  <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span> <span class="token keyword">void</span> <span class="token function">funFour</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span><span class="token punctuation">}</span>

  <span class="token comment">// 下面是一个可变参数的泛型方法</span>
  <span class="token keyword">public</span> <span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span> <span class="token keyword">void</span> <span class="token function">funFive</span><span class="token punctuation">(</span><span class="token class-name">T</span><span class="token punctuation">.</span><span class="token punctuation">.</span><span class="token punctuation">.</span>args<span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">for</span><span class="token punctuation">(</span><span class="token class-name">T</span> t <span class="token operator">:</span> args<span class="token punctuation">)</span><span class="token punctuation">{</span>
      <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>t<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>有些方法看上去像是泛型方法，实际上却不是泛型方法，像下面的这些方法就不是泛型方法：</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> test<span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">{</span>
   <span class="token keyword">private</span> <span class="token class-name">T</span> a<span class="token punctuation">;</span>
   <span class="token keyword">public</span> <span class="token function">test</span><span class="token punctuation">(</span><span class="token class-name">T</span> a<span class="token punctuation">)</span><span class="token punctuation">{</span>
     <span class="token keyword">this</span><span class="token punctuation">.</span>a <span class="token operator">=</span> a<span class="token punctuation">;</span>
   <span class="token punctuation">}</span>
   <span class="token comment">// 中并不是要给泛型方法</span>
   <span class="token keyword">public</span> <span class="token class-name">T</span> <span class="token function">getA</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
     <span class="token keyword">return</span> a<span class="token punctuation">;</span>
   <span class="token punctuation">}</span>
  
<span class="token punctuation">}</span>
</code></pre></div><h2 id="泛型集合"><a href="#泛型集合" class="header-anchor">#</a> 泛型集合</h2> <p>在文章的开头我们已经看到了泛型集合<code>List&lt;String&gt; arrayList = new ArrayList&lt;String&gt;()</code>， 其它集合泛型形式书写结构基本相似。</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token class-name">List</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">&gt;</span></span> arrayList <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Set</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">&gt;</span></span> set <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">HashSet</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token class-name">Map</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">,</span> <span class="token class-name">String</span><span class="token punctuation">&gt;</span></span> map <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">HashMap</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">,</span><span class="token class-name">String</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><h2 id="泛型接口"><a href="#泛型接口" class="header-anchor">#</a> 泛型接口</h2> <p>泛型接口和泛型类的写法基本相同。</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token comment">//定义一个泛型接口</span>
<span class="token keyword">public</span> <span class="token keyword">interface</span> itf<span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span> <span class="token punctuation">{</span>
    <span class="token keyword">public</span> <span class="token class-name">T</span> <span class="token function">fun</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>当实现泛型接口时，会这样写</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token comment">// 泛型类继承泛型接口</span>
<span class="token keyword">class</span> <span class="token class-name">Test</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span> <span class="token keyword">implements</span> itf<span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">{</span>
  <span class="token annotation punctuation">@Override</span>
  <span class="token keyword">public</span> <span class="token class-name">T</span> <span class="token function">fun</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token keyword">null</span>
  <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>


<span class="token comment">//在实现接口时我们还可以传入实参类型，来确定T的类型</span>
<span class="token keyword">class</span> <span class="token class-name">Test</span> <span class="token keyword">implements</span> itf<span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">{</span>
  <span class="token annotation punctuation">@Override</span>
  <span class="token keyword">public</span> <span class="token class-name">String</span> <span class="token function">fun</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token string">&quot;abc&quot;</span>
  <span class="token punctuation">}</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><h2 id="泛型的限制"><a href="#泛型的限制" class="header-anchor">#</a> 泛型的限制</h2> <p>我们在使用泛型时，有时会遇到限制泛型的类型范围。这个时候我们可能会用到泛型中的<strong>通配符</strong>或<strong>extends</strong></p> <h3 id="通配符"><a href="#通配符" class="header-anchor">#</a> 通配符</h3> <p>在使用通配符前我们先来看一个问题。在日常开发时我们知道<code>Ingeter</code>是<code>Number</code>的一个子类。那么<code>Test&lt;Ingeter&gt;</code>与<code>Test&lt;Number&gt;</code>是否也有这种关系？我们来看一看：</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token class-name">Test</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">T</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">{</span>
  <span class="token keyword">private</span> <span class="token class-name">T</span> a<span class="token punctuation">;</span>
  <span class="token keyword">public</span> <span class="token class-name">Test</span><span class="token punctuation">(</span><span class="token class-name">T</span> a<span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>a <span class="token operator">=</span> a<span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span><span class="token punctuation">;</span>


<span class="token keyword">public</span> <span class="token class-name">Lesson</span><span class="token punctuation">{</span>
  <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">method</span><span class="token punctuation">(</span><span class="token class-name">Test</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Number</span><span class="token punctuation">&gt;</span></span> obj<span class="token punctuation">)</span><span class="token punctuation">{</span><span class="token punctuation">}</span>

  <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token class-name">Lesson</span> le <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Lesson</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">Test</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Number</span><span class="token punctuation">&gt;</span></span> <span class="token class-name">Ntest</span> <span class="token operator">=</span> <span class="token keyword">new</span>  <span class="token class-name">Test</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Number</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token number">12</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">Test</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">&gt;</span></span> <span class="token class-name">Itest</span> <span class="token operator">=</span> <span class="token keyword">new</span>  <span class="token class-name">Test</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token number">13</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

    le<span class="token punctuation">.</span><span class="token function">method</span><span class="token punctuation">(</span><span class="token class-name">Ntest</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 这个执行编译时会通过</span>
    le<span class="token punctuation">.</span><span class="token function">method</span><span class="token punctuation">(</span><span class="token class-name">Itest</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 这个编译时 不会通过</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>通过上面的例子，我看可以看到<code>Test&lt;Ingeter&gt;</code>与<code>Test&lt;Number&gt;</code>并不是子父关系。为了解决这个问题我们可以使用<strong>通配符</strong>来修改一下</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token class-name">Lesson</span><span class="token punctuation">{</span>
  <span class="token comment">// 将方法中的类型T 替换成 ?</span>
  <span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">method</span><span class="token punctuation">(</span><span class="token class-name">Test</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span><span class="token punctuation">&gt;</span></span> obj<span class="token punctuation">)</span><span class="token punctuation">{</span><span class="token punctuation">}</span>

  <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token class-name">Lesson</span> le <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Lesson</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">Test</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Number</span><span class="token punctuation">&gt;</span></span> <span class="token class-name">Ntest</span> <span class="token operator">=</span> <span class="token keyword">new</span>  <span class="token class-name">Test</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Number</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token number">12</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">Test</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">&gt;</span></span> <span class="token class-name">Itest</span> <span class="token operator">=</span> <span class="token keyword">new</span>  <span class="token class-name">Test</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token number">13</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

    le<span class="token punctuation">.</span><span class="token function">method</span><span class="token punctuation">(</span><span class="token class-name">Ntest</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 这个执行编译时会通过</span>
    le<span class="token punctuation">.</span><span class="token function">method</span><span class="token punctuation">(</span><span class="token class-name">Itest</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 这个执行编译时也会通过</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>可以看到类型通配符是用<code>?</code>来表示。<code>?</code>在这里表示<strong>类型的实参</strong>，它代表所有类型的<strong>父类型</strong></p> <h3 id="extends与super"><a href="#extends与super" class="header-anchor">#</a> extends与super</h3> <p>在使用泛型时，我们有时候会想对类型实参进行限制，比如:</p> <ul><li>泛型中的实参类型只能是<code>Number</code>类及其子类（这种叫做<strong>类型通配符上限</strong>）</li> <li>泛型中的实参类型只能是<code>Number</code>类及父类（这种叫做<strong>类型通配符下限</strong>）</li></ul> <p>类型通配符上限我们使用<strong>extends</strong>来限制</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">method</span><span class="token punctuation">(</span><span class="token class-name">Test</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span> <span class="token keyword">extends</span> <span class="token class-name">Number</span><span class="token punctuation">&gt;</span></span> obj<span class="token punctuation">)</span><span class="token punctuation">{</span><span class="token punctuation">}</span>
</code></pre></div><p>类型通配符上限我们使用<strong>super</strong>来限制</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">method</span><span class="token punctuation">(</span><span class="token class-name">Test</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token operator">?</span> <span class="token keyword">super</span> <span class="token class-name">String</span><span class="token punctuation">&gt;</span></span> obj<span class="token punctuation">)</span><span class="token punctuation">{</span><span class="token punctuation">}</span>

 <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token class-name">Lesson</span> le <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Lesson</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">Test</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">&gt;</span></span> <span class="token class-name">Stest</span> <span class="token operator">=</span> <span class="token keyword">new</span>  <span class="token class-name">Test</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">String</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token string">&quot;12a&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">Test</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Object</span><span class="token punctuation">&gt;</span></span> <span class="token class-name">Otest</span> <span class="token operator">=</span> <span class="token keyword">new</span>  <span class="token class-name">Test</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">Test</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">&gt;</span></span> <span class="token class-name">Itest</span> <span class="token operator">=</span> <span class="token keyword">new</span>  <span class="token class-name">Test</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Integer</span><span class="token punctuation">&gt;</span></span><span class="token punctuation">(</span><span class="token number">13</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

    le<span class="token punctuation">.</span><span class="token function">method</span><span class="token punctuation">(</span><span class="token class-name">Stest</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 这个编译时会通过</span>
    le<span class="token punctuation">.</span><span class="token function">method</span><span class="token punctuation">(</span><span class="token class-name">Otest</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 这个编译时会通过</span>
    le<span class="token punctuation">.</span><span class="token function">method</span><span class="token punctuation">(</span><span class="token class-name">Itest</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token comment">// 这个编译时不会会通过</span>
  <span class="token punctuation">}</span>
</code></pre></div><p><strong>参考文献</strong></p> <ul><li><a href="https://www.cnblogs.com/lwbqqyumidi/p/3837629.html" target="_blank" rel="noopener noreferrer">Java总结篇系列：Java泛型<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li> <li><a href="https://www.runoob.com/java/java-generics.html" target="_blank" rel="noopener noreferrer">Java 泛型<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li> <li><a href="https://www.cnblogs.com/coprince/p/8603492.html" target="_blank" rel="noopener noreferrer">java 泛型详解-绝对是对泛型方法讲解最详细的，没有之一<span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></li></ul></div> <footer class="page-edit"><!----> <div class="last-updated"><span class="prefix">最近更新时间:</span> <span class="time">7/2/2021, 11:27:27 AM</span></div></footer> <div class="page-nav"><p class="inner"><span class="prev">
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    </span></p></div></div> <div class="floatSilder hidden-sm-and-down"><div class="floatSilderWrapper" data-v-4100c7a9><div class="topTitle" data-v-4100c7a9>Java-泛型</div> <div class="contentNav" data-v-4100c7a9><ul class="nav" data-v-4100c7a9><li class="title" data-v-4100c7a9><a href="#前言" data-v-4100c7a9>前言</a></li><li class="title" data-v-4100c7a9><a href="#泛型类" data-v-4100c7a9>泛型类</a></li><li class="title" data-v-4100c7a9><a href="#泛型方法" data-v-4100c7a9>泛型方法</a></li><li class="title" data-v-4100c7a9><a href="#泛型集合" data-v-4100c7a9>泛型集合</a></li><li class="title" data-v-4100c7a9><a href="#泛型接口" data-v-4100c7a9>泛型接口</a></li><li class="title" data-v-4100c7a9><a href="#泛型的限制" data-v-4100c7a9>泛型的限制</a></li><li class="subTitle" data-v-4100c7a9><a href="#通配符" data-v-4100c7a9>通配符</a></li><li class="subTitle" data-v-4100c7a9><a href="#extends与super" data-v-4100c7a9>extends与super</a></li></ul></div></div></div></div> </main></div></div><div class="global-ui"><!----></div></div>
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